Modulation Transformer Calculation

Some years ago I wrote on this subject. If you look at the connection diagrams for any multi-match modulation transformer, it is apparent that for any given connection choice, there will be a variety of primary and secondary impedances supported. Therefore I make the point that turns ratio is the primary concern before looking at the impedances.

Let me try to make an example to see if this helps. You built a home brew transmitter using a pair of 6146's that you want to plate modulate. Your power supply provides 600V, and is beefy enough to supply both the RF amplifier, and the modulator.

So let me start this process for the plate modulator. You want the ability to modulate fully, and with a little extra headroom. So lets say you want to be capable of 110% on positive peaks.

So to get to 110%, multiply 1.1 X 600 = 660v. The modulated B+ goes from 600 to 600 +660 = 1260v peak at 110% modulation. On the other way down it will go to -60v (over mod!).

For modulators you have a pile of 807's, so lets explore them. You also want high quality audio, so you want to bypass the interstage transformer idea, and instead you want to build a phase inverter, and R-C couple the audio to the 807 grids. This means class Ab1 operation since you cannot provide any grid current. With class Ab1, the plate voltage minimum occurs when the peak AF grid voltage matches the grid bias amplitude. If the bias were set to -33v, and peak AF swing brings the G1 to 0v. You cannot go positive since grid current would flow.

Next look up the 807, and look at the plate characteristic graph. At zero bias, the plate voltage saturation is about 50v, but that is the hard clip point. Back off from that, and lets say the plate voltage minimum is 150 volts. So this means the 807 plate can each swing from B+ (600v) to 150v (Eb min) to 1050v (Eb (max), or +/- 450v. Since this is Push Pull, one 807 at Eb min of 150, and the other will be at Eb max of 1050. So we make 900v peak across the primary of the modulation transformer.

We said earlier that we need 660 v peak across the secondary for 110% modulation, so the voltage ratio is 900/660 = 1.36:1. The voltage ratio is the same as the turns ratio.

So lets say the 6146's run at 600v @ 200ma. The RL = 600/.2 = 3000 ohms.
With a secondary load impedance of 3000 ohms, and a mod tranny with a 1.4:1 turns ratio, the primary plate to plate load impedance is:

1.4^2 X 3000 = 6000 ohms.

It looks like a pair of 807's will do fine. But what if it worked out to 3500 ohms instead. that is too low for those 807's, but two pair in push pull parallel would be fine.

If you concentrate on impedances only, you could end up matching impedances when the turns ratio is way off. This could limit the maximum modulation percentage to something below 100%.

Modulation transformers simply provide an air gap between the final modulator and the RF amplifier. The only thing that matters is the transformer's ratio. Always go by the ratio and not the impedance.

For years now AM'ers that want to build a plate modulated rig with a push pull modulator have had to scrounge for a decreasing supply of old modulation transformers. One exception is the "big-iron" tube type broadcast transmitters that are being scrapped. One recent development with smaller AM Ham modulations is from Hammond Transformers that is increasingly offering the Peter Dahl line of transformers. This is cool...not cheap, but cool. A brand new Viking 500 modulation transformer for $390. This ran a pair of 811's in push pull modulating a single 4-400 RF tube.